This application claims the priority of German Patent Application Serial No. 102 56 151.6, filed Nov. 29, 2002, pursuant to 35 U.S.C. 119(a)-(d), the subject matter of which is incorporated herein by reference.
The present invention relates a method to produce weakened zones by means of an ultra sound cutting process and in particular to a method for produce weakened zones in plastic trim parts by means of an ultra sound cutting method.
Plastic trim parts which are provided with weakened zones or designated breaking lines are frequently used as panels covering the interior of cars and automobiles. They are for example also used as optical covers for security systems, such as in devices for airbags in an automobile. Such weakened zones should be advantageously provided in the plastic trim parts so that they are invisible and remain stable over the course of a long time, up to many years. These weakened zones can be grooves cut into the plastic trim parts by various methods.
From the prior art, the use of ultra sound for producing such grooves in plastic trim parts is known for example from U.S. Pat. No. 6,440,051 where a method of using an ultra sound welding apparatus is disclosed. This method is used to produce perforations in trim parts by means of a continuous process. However, the process does not produce grooves of weakened zones in the plastic trim part, but is limited to a perforation of the series of strata (12) and the perforations are not invisible. A decorative layer (12) as well as a foam layer (14) are perforated, but not the layer (15) designated as the barrier layer (15). These perforations are needed for decorative purposes only, since they are applied on the visible side of the interior paneling.
In addition to the above described perforations, weakened zones which are configured as designated breaking lines are provided. These weakened zones extend through the base layer and partially into the series of layers (12). The weakened zones are applied by means of an ultra sound welding apparatus, by means of a heated or non-heated cutting knife or a high frequency cutting device. However, with this method only V-shaped cuts as shown in FIG. 3 can be realized. However, such a V-shaped cut can be of advantage only if the displaced material from the cutting operation can be securely removed through the cutting motion.
Furthermore, producing weakened zones in expanded polypropylene (EPP) are known from U.S. patent publication No. 2001/0045728A1. The weakened zones there are also provided in the form of V-shaped grooves. According to this teaching, the V-shaped weakened zone is configured as a recess and not as a groove. It is particularly emphasized that the V-shaped weakened zone is produced by a pressure process. By means of the pressure process a higher density is allegedly realized in the area of the V-shaped weakened zone than in the unweakened zone of the trim part in order to provide local stability to the weakened zone. However, in this prior art patent the ripping seam of an airbag seam is not invisible at the side which is visible to the passenger, i.e. the viewing side, nor that it is a desired property to be retained for many years of the vehicle""s use. Also, the V-shaped configuration of the cutting profile renders it is visible on the viewing side of the plastic trim part, which is undesirable and is detected in heat storage tests.
EP 0711627 shows the configuration of a cut (e.g. FIGS. 9-12) relating to perforations which are produced by means of laser cutting methods. According to this prior art reference, wider cuts are considered advantageous especially when cuts are configured in a line since these cuts are subsequently designed to be filled with material and thus the cut configurations are not designed for narrow cuts with a size of width of up to 1 mm.
Furthermore, use of a cutting means operated with ultrasound is disclosed in DE 44 09 405. However, the ultra sound cutting is applied to a series of layers of polyurethane foam and a plastic carrier. It has been found that especially for these material combinations the method described in the reference is less advantageous than a laser cutting method also disclosed there. The laser energy causes the plastic section to evaporate thereby eliminates the problem of the discharged material. A solution to the problem of the discharge or cut-away material using ultra sound is not disclosed.
Accordingly, none of the afore-described prior art provides a solution for producing a permanent and invisible tear seam for example for an airbag or another type of plastic part using weakened zone in a multilayered plastic trim part, where it is required that the weakened zones extend through the carrier-, barrier- and foam layers into the decorative layer.
The methods for producing invisible air bag tear seams currently used, such as milling, laser cutting, or laser perforation, when used with expanded poly-propylene (EPP), result in additional problems when carrying out the cuts due to bonding of the cut material which thus ends up in an unsmooth cutting profile. It is therefore also virtually impossible not to end up with essentially a V-shaped cutting profile.
As series of tests have demonstrated, the known prior art does not provide the basis for solving the drawbacks and shortcomings as discussed here since neither milling nor laser cutting or laser perforation have provided the desired results, since the discharge of material from the cutting site is only possible when the configuration of the cut essentially approximates a V-shape.
It would therefore be desirable and advantageous to provide an improved method to provide invisible weakened zones in plastic trim parts and to obviate prior art shortcomings by providing the plastic trim parts with weakened zones which remain invisible even after a long period of time, such as several years and even under changing temperature conditions.
According to one aspect of the invention, a weakened zone is produced in plastic parts in the form of a cut of very small width, and consequently prevent that discharged material from the cutting action accumulates so that it is visible at a visible side of a multilayered plastic part.
According to another aspect of the present invention, a method for producing weakened zones in a plastic trim part is provided, wherein the plastic trim part has a carrier layer and a decorative layer, including the steps of inserting the decorative layer into a foaming tool and coating the decorative layer with a foam layer, whereby the decorative layer is attached to a carrier layer of expanded poly propylene or between the decorative layer and the carrier layer a foam layer extends and each of the carrier layer and the foam layer are having a depth dimension, subsequently bringing an ultra sound cutting device in contact with the carrier layer and the foam layer thereby cutting into the carrier layer and the foam layer and producing a cut through the carrier layer and the foam layer and extending along a longitudinal extension with a width of up to 1 mm.
The present invention resolves prior art problems by providing a method of cutting, wherein the amount of discharged material is so small that the cut is no longer required to be in a V-shape.
Thus, if the cut has a cross section of such a small size that it can hardly be detected by plain eye sight, the material discharge is thereby not only reduced to a minimum, but the cut itself hardly weakens the plastic material even if forces are applied which are generated under normal use of the plastic trim part. These forces are at least partially applied in a direction normal to the cutting plane, whereby the cut either closes by itself or behaves in same manner as its surrounding area or, the cut only widens slightly. Thereby, no large forces are impacting the decorative layer. However, should the airbag release the forces impacting the cut be directed in a direction parallel to the cutting surface, the remaining part of the unweakened cross section can be cleanly severed, without the formation of break-down material.
With the method according to the present invention of producing weakened zones in plastic parts, it is possible to produce weakened zones in plastic parts utilized for example for the interior of a vehicle, wherein the weakened zones which will not be visible on the visible side and which can be retained in that condition for several years.
Since the ultra sound cut does not damage the material of the carrier layer when the carrier layer is made from a special material such as for example polypropylene, neither bonding nor formation of any residue or unsmooth surfaces on the cutting surface are the consequence. Thus, the ultra sound cutting process permits the production of very smooth cutting surfaces.
Since the residue material in the cutting profile is thus eliminated, very precise cuts can be realized. These cuts can be of any depth so that in an advantageous embodiment not only the carrier layer but also the foam layer and parts of the decorative layer can be cut. These cuts can also be produced within any time frame after the manufacturing of the plastic part. As a result, there is no need for coupling the manufacture of the plastic part with the cutting apparatus but can be separately carried out by separate installations in different locations. This allows the separate processing steps to be carried out for example at the time when the plastic part is mounted into a vehicle.